Driving apparatuses used in electric vehicles for railways have an operation mode called dynamic braking in which regenerative energy generated by a motor for driving wheels is consumed by brake resistance when a brake is activated. In this mode, a conduction ratio calculated on the basis of a filter capacitor voltage in a side of a direct-current voltage input of an inverter for driving the motor is compared with a brake-chopper triangular carrier wave. Based on a comparison result thereof, a gate signal for a brake-chopper switching circuit is generated thereby to control conduction conditions.
Harmonic components which are generated when such a brake-chopper switching circuit operates sometimes cause inductive interference with an electric generator system. As relevant prior art techniques to reduction of harmonic components, Jpn. Pat. Appln. KOKAI Publications No. 9-140165 and No. 2006-67638 have been disclosed. These publications both describe reduction of harmonic waves which are generated by a converter.
In a conventional vehicle system, a brake-chopper switching circuit uses a constant carrier frequency, and therefore generates harmonic components at a particular frequency. FIG. 17 shows simulation results concerning harmonic components generated by switching of a conventional brake-chopper switching circuit. A power generation system is under limitations of limit values to harmonic components for various signal systems, for example, because communications are established by using rails. A curve 100 denotes such a limitation value. A waveform 111 represents a spectrum waveform of a harmonic wave which was generated from a brake-chopper switching circuit when one inverter circuit is used. A waveform 112 represents a spectrum waveform of a harmonic wave which was generated from the brake-chopper switching circuit when a string of four engine vehicles (including eight inverter circuits) was used. In this example, the brake-chopper switching circuit has a switching frequency fBCH of 327 Hz, and a motor rotation frequency was 8.6 Hz.
At the switching frequency fBCH, a ripple 113 occurred and suggests a high-frequency component which was generated by switching the brake-chopper switching circuit. Ripples 114 and 115 respectively suggest secondary and tertiary high-frequency components which were generated from the brake-chopper switching circuit. Other ripples suggest high-frequency components which were generated by inverters. Thus, a margin of the ripple 113 of a high-frequency wave which was generated by the brake-chopper switching circuit was small (i.e., a margin value from a ripple peak to a most strict limitation value).
Although the switching frequency fBCH of the brake-chopper switching circuit is constant, a frequency of ripples generated by an inverter varies in proportion to a vehicle speed. Accordingly, a harmonic component of a particular frequency fBCH and a harmonic component, which is generated by superposing with a harmonic component generated from an inverter, may cause inductive interference with communication channels of a power generation system.
In an embodiment of the invention, plural brake-chopper switching circuits are provided in an electric-vehicle driving apparatus in which regenerative energy is consumed by a brake resistor. Harmonic components generated by the brake-chopper switching circuits are reduced by changing phases or frequencies of triangular carrier waves of the respective switching circuits. As a result, stable communications can be performed through a power generation system (rails).